Combustion apparatus for and method of burning ash-forming liquid fuel



July 21, 1959 HQDGE 2,895,293

COMBUSTION APPARATUS FOR AND METHOD OF BURNING v ASH-FORMING LIQUID FUELFiled May 14, 1956 3 Sheets-Sheet l L' AttorneS S 3 Sheets-Sheet 2 R. I.HODGE ASH-FORMING LIQUID FUEL July 21, 1959 COMBUSTION APPARATUS FOR ANDMETHOD OF BURNING Filed May 14, 1956 n m? m/ v m g m4 R y Q m 2. WWW OmZ i -w, 0mm vmm 1 E w 0% 1 0M1 m no no 3 m 2m 2 +0 no o m me Z J mm on mo mm nmm I I! I \I I f I!) I ll!- wm E Q 21, 1959 v R HQDGE 2,895,293

COMBUSTION APPARATUS FOR AND METHOD OF BURNING ASH-FORMING LIQUID FUELFiled May 14, 1956 5 Sheets-Sheet 3 Unite tates atent Ronald InglisHodge, Church Crookham, near Aldershot,

England, assignor to Power Jets (Research and Development) Limited,fLondon, England, a British company This invention relates to combustionapparatus for burning ash-forming liquid fuel, particularly though notnecessarily exclusively fuel oil such as residual fuel oil which oncombustion yields an ash containing substances such as vanadium andsodium compounds which at elevated temperatures, e.g., 650 C. and above,give rise to accelerated corrosion of metallic parts with which the ashcomes into contact. Such accelerated corrosion is a very serious problemin gas turbines operating on low grade fuel oils. it is an object ofthis invention to provide a method of reducing such corrosion.

It has been found that by controlling combustion in combustion apparatususing fuel oils of the type indicated above so that the gases resultingfrom the combustion of the fuel include a proportion of unburnt carbon,deposition of ash on and consequent corrosion of the parts upon whichthe ash would normally be deposited can be reduced or eliminated. Itwould appear that the dry carbon particles tend to absorb the wet andsticky ash, and so much of the ash will be discharged with asubstantially reduced risk of its being deposited on, for example, theturbine blades.

The present invention stems from the realization that in order to effectcombustion control in the manner indicated above, it is necessary tocontrol the size of the liquid fuel droplets burnt in the apparatus,since if droplets of a wide range of sizes are burnt, the smaller oneswill be burnt away completely to form ash only, While the larger onesare only partly burnt. Thus in the case of the smaller droplets, therewill be no unburnt carbon associated with the ash resulting from thecombustion of these droplets and this free ash can give rise tocorrosion. On the other hand if combustion is controlled so that eventhe smallest droplets are not completely burnt, the exhaust solidsresulting from the combustion of the larger droplets will contain a muchgreater proportion of unburnt carbon, and the resultant loss ofcombustion efliciency may be unacceptable.

Accordingly, the present invention provides combustion apparatus forburning ash-forming liquid fuel in which fuel droplets, substantiallyall of which are within a predetermined range of size, are introducedinto the combustion zone of the apparatus and are partially burnttherein so that the combustion gases contain a proportion of unburntcarbon which absorbs the fuel ash.

The invention further provides combustion apparatus as aforesaidcomprising a fuel injector discharging a spray of fuel droplets ofvarious sizes, and means for separating fuel droplets of sizes outsidesaid pre-deter mined range from the remainder of the spray before itenters the combustion zone.

According to a feature of the invention, only the larger droplets, beingsubstantially all above a pre-determined size, are introduced into thecombustion zone. A fluid stream may be led into contact with the fuelspray so that it causes smaller droplets of less than saidpre-determined size to be separated from the remainder of the spray.

ice

The injector may be located in a duct leading at one end to thecombustion zone, and open at the other end to receive an air streamflowing to the combustion zone, said stream carrying the remainder ofthe spray to the combustion zone, and the injector may discharge agenerally conical fuel spray and the fluid stream may cross the path ofthe spray. There may be a collector pipe, the open end of which receivesthe separated droplets.

In combustion apparatus of the type with which the present invention isconcerned, it is necessary to make provision for a stabilized flame zonein which combustion is to be initiated, and it is desirable to ensurethat there shall be no deposition of unburnt carbon or of ash in thiszone; the use of a conventional flame stabilizing baffle is therefore tobe avoided. Thus there may be a constrictionat the entry to thecombustion zone, combustion air being supplied to the combustion zonethrough the constriction with swirl about the axis thereof so thatre-circulation along the axis of the constriction is set up and astabilized combustion zone formed.

Difficulty may arise in that the fuel may contain a considerableproportion of comparatively volatile constituents. The conditions whichallow the rapid evaporation and efficient combustion of theseconstituents in known combustion chambers lead to a loss of control overthe burning of the non-volatile constituents, and this lowers theeffectiveness of the method of reduction of corrosion indicated above.It is desirable therefore to burn the volatile constituents in a mannerwhich gives rise to the minimum disturbance in the flow through the zonein which these constituents are burnt. Accordingly, the fuel forcombustion may be introduced into the upstream end of a vaporizing ductleading to the entry to the combustion zone, in which duct at least partof the volatile constituents of the fuel are vaporized.

The combustion apparatus may comprise a vortex combustion chamber with acentral axial outlet in one of its side walls and a generallytangentially facing inlet in its peripheral wall to which is connected apro-combustion chamber, the fuel to be burnt being introduced into andignited in the pre-combustion chamber and continuing to burn while beingcarried around the axis of the vortex chamber in a spiral vortex path.Air may be admitted to mix with and chill the combustion gases at theoutlet of the vortex chamber.

One embodiment of the invention will now be described by way of examplewith reference to the accompanying drawings,of which:

, Figure 1 is a diagrammatic view of a gas turbine plant incorporatingcombustion apparatus of the spiral vortex type shown in longitudinalcross-section.

Figure 2 is a transverse cross-sectional view of the pcripheral part ofthe combustion apparatus shown in Figure 1. i

Figure 3 is a section on the line IIIIII in Figure 2.

Figure 4 shows a detail of the arrangement of Figure 2 to a longerscale.

In Fig. 1 a gas turbine plant comprises a compressor 1, a turbine 2driving the compressor through a shaft 3 and also driving a load such asan alternator 4. The compressor is connected to supply compressed airthrough duct 5 to combustion apparatus of the spiral vortex type and theturbine is connected to receive hot combustion gases through duct 6 fromthe combustion apparatus whereby it is driven.

The combustion apparatus is of the spiral vortex type operating on theprinciples described in British Patent No. 639,468 and is structurallysimilar in some respects to the apparatus described in British PatentNos. 719,379 and 719,380. It comprises an outer casing 11 having aperipheral. wall 11a and two generally frusto-conical end walls 11b and110, and a vortex combustion chamber 12, likewise having a peripheralwall 12a, and end walls 12b and 120, mounted in the casing by means ofsupports 13 which maintain the chamber concentric with the casing andrestrain it against bodily movement but permit differential thermalexpansion. Such supports are shown in the two last mentioned patents.The combustion chamber has a lining 14 of a refractory material. The endwall 12b of the combustion chamber is formed with a central outletaperture 15 from which extends an axial outlet duct 16 to which isconnected the duct 6 for conveying hot combustion gases to the turbine2, and its other end wall 120 is also formed with a central aperture 18from which a tube 19 extends axially into the interior of the chamber.

Its peripheral wall 12a is formed with two diametrically oppositetangentially facing generally segment-shaped inlet apertures (one ofwhich is shown at 17) to which are connected generally tangentiallyextending pre-combustion chambers 31 as shown in Fig. 2, thepre-combustion chambers and the vortex combustion chamber constitutingthe combustion zone of the apparatus.

The outer casing 11 similarly has a central aperture in its end wall 11band a tubular extension 20 extending axially therefrom around the outletduct 16. The end of the annular space between the extension 20 and duct16 is closed by an end wall 21, and an air inlet duct 22 connected tothe duct 5 from the compressor 1 leads laterally into this space. Thespace is further divided radially by an axially extending tubular wall23 While the inlet duct 22 is split into two passages of unequalcrossectional area by a partition 24, the larger passage communicatingwith the annular space between the extension 20 and the wall 23 and thesmaller with the annular space between the wall 23 and duct 16.

The peripheral walls 11a of the casing is further formed with twodiametrically opposite apertures to which are connected generallytangential extensions 32 closed at their ends and constituting aircasings enclosing the precombustion chambers 31 (see Fig. 2).

In operation, the air supply entering the inlet duct 22 is split bypartition 24 into two streams. The smaller stream enters the spacebetween wall 23 and duct 16 and enters the later through rows of holes16a The larger stream enters the space between the casing 11 andcombustion chamber 12, thus cooling the latter, and is further divided,some passing into the air casings 32 and so into the pre-combustionchambers 31 as combustion air, and the remainder entering the combustionchamber 12 axially through apertures 18. Butterfly valves 25, 26,separately operable by levers 25a, 26a, are provided in the inlet duct22 for controlling the air streams on each side of the partition 24. Theflow through aperture 18 is controlled by a further valve 27 mounted ona threaded rod 28 carried in the end wall 110 of the casing and operableby hand wheel 29. By adjustment of these valves, the magnitudes of thevarious air streams can be varied relatively to one another.

As shown in Fig. 2, the pre-combustion chamber 31 comprises acircular-section portion 31a, an outlet portion 31b of segmental crosssection (conforming to the shape of the inlet 17 to the vortexcombustion chamber as shown in Fig. 1) and a transition portion 310. Thechamber 31 is enclosed by a similarly shaped outer sheath 33 having aflared end 33a and annularly spaced therefrom by spacers 34 to define anannular passage for cooling air. \The sheath has an external flange 33bby which it is secured to the peripheral wall 12a of the vortexcombustion chamber. At the upstream end of the pre-combustion chamber 31there is a constriction of substantially venturi shape having a throat35 of smaller diameter than the circular-section portion 31a. The throatis connected to the latter by a short frusto-conical wall 36 and thereis a similar short frusto-conical wall 61 converging to the upstream endof the throat. Extending upstream from this last mentioned wall is atubular vaporizing duct 62. Ali annular air inlet is formed in the wallof this duct immediately upstream of the constriction in which aremounted swirl vanes 63 (see Fig. 3.). The inlet might be replaced by anumber of separate tangentially directed inlets or by a 360 volute, thearrangement in any case being such that the air can be admittedtherethrough with swirl about the axis of the duct. At the upstream endof the vaporizing duct 62 there is an annular air inlet defined by twofrusto-conical walls 64-, 65, the wall 65 being supported from the -wall64 by axially and radially extending splitters 66. Mounted within thevaporizing duct are a series of telescoping liners 67.

Within the inner Wall 65 is mounted a tube 71 open at both ends andconstituting a duct through which further air can flow from theextension 32 into the vaporizing duct. As shown in Fig. 4 two furthertubular members 72, 73 are mounted co-axially within the tube 71, thelatter being connected to a curved collector pipe 74 leading to theexterior of the vaporizing duct. A liquid fuel injector 75 is mounted inthe entry to member 73 within a faired shroud 76. This injector is ofthe known swirl atomizing type having an internal swirl chamber to whichfuel is supplied through one or more tangential swirl ports and fromwhich the fuel is discharged through an axial discharge nozzle 75a as agenerally conical spray of atomized fuel made up of droplets of varioussizes. Liquid fuel, being a residual fuel oil of the type referred toabove, is supplied to this injector from fuel tank 45 by a fuel pump 44(as in the embodiment of Fig. 2.), through a pipe 77 nested within thepipe 74-. A fairing 78 is mounted Within the tubular member 72co-axially with the injector, the adjacent ends of the shroud 7 6 andfairing 78 being shaped to form between them an annular gap conformingsubstantially to the cone angle of the fuel spray. The gap 79 betweenthe members 72, 73 is also approximately aligned with the conical fuelspray as will be shown below.

In operation air from extension 32 enters the tubular member 72, isdiverted outwardly by the fairing 78 and so flows into the annular spacebetween the tubular member 73 and shroud 76. In so doing, the air streamcrosses the path of the conical fuel spray. This spray is made up offuel droplets of various sizes, and the smaller ones are diverted by theair stream and carried thereby into the tubular member 73 and so awaythrough pipe 74. The larger particles on account of their greaterinertia are not diverted from their original path to such a great extentand so are able to reach the gap 79. A further air stream from theextension 32 flows between the tube 71 and the tubular member 72 andthis stream carries the larger fuel droplets into the vaporizing duct62. The trajectories of the smaller and larger droplets are indicated bythe broken lines X and Y respectively.

The pipe 74 is connected to the inlet of a cyclone separator 47 in whichthe fuel is separated from the air stream. The separated fuel iscollected in trap 48 from which it drains through pipe 49 back to thefuel tank 45. The air is discharged through outlet duct 50 to a regionof lower pressure, e.g., to atmosphere, or to the inlet of compressor 1,or to some other low pressure region of the combustion apparatus or ofthe plant, the pressure differential promoting air flow down pipe 74 toremove the smaller droplets. If required, suction may be applied tooutlet duct 50 to accelerate the air flow, but normally this will not benecessary as the air supply to the apparatus will be under pressure.

By suitable choice of the relative locations and dimensions of the partsof the apparatus, the air velocities, and the pressure differentialpromoting flow down tube 74 in relation to the range of size of the fueldroplets discharged from the injector 75, it can be arranged thatsubstantially only those droplets of above a pre-determined size areallowed to enter the vaporizing duct and the pre-combustion chamber. Inthe embodiment illustrated, gauzes 80 are provided in the inlet totubular member 72 to reduce the velocity of the air stream flowingtherethrough, while the tube 71 is convergent from its inlet end toaccelerate the air stream flowing into the vaporizing duct. The end ofthe tubular member 72 which forms one edge of the gap 79 is aligned withthe undeflected path of the fuel spray indicated by the broken line Z.

It will be noted that air flowing through pipe 74 serves to some extentto protect the fuel in pipe 77 against overheating.

The air entering the vaporizing duct is pre-heated by compression andmight in some arrangements also be heated by passage through a heatexchanger, and thus serves to effect vaporization of the more volatileconstituents of the fuel oil droplets discharged into the vaporizingduct 62. The duct is long enough having regard to its cross-sectionalarea, the velocity, mass flow and temperature of the air, and thequantity and composition of the fuel that the major part of the volatileconstituents of the fuel are vaporized in the duct before they reach itsdownstream end. Air admitted progressively through the annular gapsbetween successive liners 67 serves to prevent fuel droplets coming intocontact with the duct *wall with the consequent risk of carbondeposition thereon.

At the downstream end of the vaporizing duct, further air is admittedwith a strong swirl about the duct axis through swirl vanes 63. Thisswirling air passes through the constriction at the upstream end of thepre-combustion chamber 31, its velocity and quantity being such inrelation to the size and configuration of the constriction as to giverise to re-circulation along the axis thereof. The fuel is ignited bymeans of a torch igniter 53 or other ignition device, and stablecombustion of the volatile constituents of the fuel takes place in thestabilized flame zone set up by the re-circulation. It is to be notedthat there is no conventional flame stabilizing baffle on which carbondeposition might occur.

The burning volatile constituents give rise to a flame in thepro-combustion chamber 31 which serves to ignite the non-volatileconstituents of the fuel. These are carried into the vortex combustionchamber 11 and are burnt therein as they are carried around the axis ofthe chamber in a spiral vortex path, progressively approaching thecentral outlet 15 as they are burnt away. Since the size of the fueldroplets burnt is controlled, substantially all being above apre-determined size, they will all be burnt away to substantially thesame extent when they reach the outlet. Thus by appropriate design ofthe vortex combustion chamber, it may be arranged that the fuel dropletsare only partially burnt and a pre-determined proportion of unburntcarbon remains associated with the fuel ash from each fuel droplet whenit reaches the outlet. To ensure incomplete combustion and so to obtaina certain proportion of unburnt carbon in the combustion gasesdischarged through the outlet duct 16, the gases are mixed with andchilled by the air entering through the tube 19 and holes 16a in theoutlet duct 16. Combustion is thus stopped before it is complete and theresultant free carbon in the exhaust solids serve to reduce or eliminatedeposition of ash on the turbine blades and elsewhere in the gas turbineas mentioned above. This chilling air constitutes the dilution airrequired to reduce the combustion gas temperature to a value which canbe supported by the turbine, the quantity of air entering thepre-combustion chambers 33 being sufficient to support combustion offuel.

In some cases, the proportion of volatile constituents vaporized in thevaporizing duct 62 will be less than in the example described above;e.g., the proportion vaporized may be or less, depending upon thecomposition of the fuel oil used. The remainder of the volatileconstituents will then be vaporized in the pre-combustion chamber. Thevaporizing duct 62 and swirl vanes 63 might be omitted altogether, i.e.,the tube '71 and the associated fuel supply and droplet separationarrangement could be used in conjunction with a pre-combustion 6 chamberassembly as described-in a copending application Serial No. 584,577 inthe name of J. A. Gardiner filed on the same day as the presentapplication.

The assembly of vaporizing duct, pre-combustion chamber and vortexcombustion chamber described above could also be used in conjunctionwith the fuel supply and droplet separation arrangement described insaid copending application, or with that described in a copendingapplication Serial No. 584,630'in the name of M. V. Herbert also filedon the same day as the present application.

I claim:

1. A combustion method for burning ash-forming liquid fuel thatcomprises the steps of establishing a combustion zone; discharging saidfuel as a spray of droplets of various sizes into a region outside saidcombustion zone; separating from the spray substantially all thosedroplets of sizes less than a pre-determined size, which separateddroplets would, under the conditions of combustion prevailing in saidzone, be substantially completely burned; introducing the. remainder ofthe droplets into the combustion zone; igniting said remainder of thedroplets in said zone; partially burning said ignited droplets in saidzone so that a proportion of unburnt carbon associated with fuel ash isformed; and discharging said carbon with the combustion gases from saidzone.

2. A combustion method for burning ash-forming liquid fuel thatcomprises the steps of establishing a combustion zone and a vaporizingzone; discharging said fuel as a spray of droplets of various sizes intoa region outside said zones; separating from the spray substantially allthose droplets of sizes less than a pre-determined size, which separateddroplets would, under the conditions of combustion prevailing in saidzone, be substantially completely burned; introducing the remainder ofthe droplets into the vaporizing zone; heating said remainder of thefuel droplets in the vaporizing zone to vaporize at least part of thevolatile constituentsof the fuel droplets; introducing the vaporizedfuel and said droplets into the combustion zone; igniting said vaporizedfuel and the droplets in the combustion zone; partially burning thenon-volatile constituents in the combustion zone so that a proportion ofunburnt carbon associated with fuel ash is formed, and discharging saidcarbon with the combustion gases from said zone.

3. Combustion apparatus for burning ash-forming liquid fuel comprisingmeans defining a combustion zone; a duct leading at one end to saidcombustion zone; a fuel injector mounted within said duct, said injectorbeing ofa type which discharges said fuel as a spray of droplets ofvarious sizes; means for separating from said spray substantially allthose droplets of sizes less than a predetermined size, which separateddroplets would, under the conditions of combustion prevailing in saidzone, be substantially completely burned; means for removing saidseparated droplets from the duct; means [for supplying air to the otherend of the duct to carry the remainder of the droplets into thecombustion zone; an igniter for igniting said remainder of the dropletsin the combustion zone; said zone being shaped and dimensioned so thatsaid ignited droplets are only partially burnt therein so that aproportion of unburnt carbon associated with fuel ash is formed, andhaving an outlet for combustion gases.

4. Combustion apparatus for burning ash-forming liquid fuel comprisingmeans defining a combustion zone; a duct leading at one end to saidcombustion zone; a fuel injector mounted within said duct, said injectorbeing of a type which discharges said fuel as a spray of droplets ofvarious sizes; means for directing a fluid stream in a directiondifferent from that of and into contact with said spray at such avelocity in relation to the mass, dimensions and velocity of thedroplets as to separate and carry away therefrom substantially all thosedroplets of sizes less than pre-determined size; means to receive saidfluid stream with the droplets carried away thereby; means for supplyingair to the other end of said duct to carry the remainder of the dropletsinto the combustion zone; an igniter for igniting said remainder of thedroplets in the combustion zone; said zone being shaped and dimensionedso that said ignited droplets are only partially burnt therein so that aproportion of unburnt carbon associated with fuel ash is formed, andhaving an outlet for combustion gases.

5. Combustion apparatus for burning ash-forming liquid fuel comprisingmeans defining a combusion zone; a duct leading at one end to saidcombustion zone; two coaxial open-ended tubular members axiallyseparated by an annular gap mounted within the duct and definingtherewith an annular passage; a fuel injector mounted coaxially withinthat tubular member nearer the combustion zone end of the duct with aradial clearance therearound, said injector being of the type whichdischarges fuel as a generally conical spray of droplets of varioussizes, said annular gap being substantially aligned with said spray; acollector pipe having an open end connected to said last-mentionedtubular member; means to supply air to the end of the other tubularmember remote from the combustion zone to impinge upon the fuel spray atsuch a velocity in relation to the mass, and dimensions and velocity ofthe droplets as to carry the fuel droplets of less than a predeterminedsize into the radial clearance around the injector and so into thecollector pipe and to the end of the duct remote from the combustionzone to carry the remainder of the droplets into the combustion zone; anigniter for igniting said remainder of the droplets in the combustionzone; said zone being shaped and dimensioned so that said igniteddroplets are only partially burnt therein so that a proportion ofunburnt carbon associated with fuel ash is formed, and having an outletfor combustion gases.

6. Combustion apparatus according to claim comprising means for causingan air flow down said collector pipe from said open end.

7. Combustion apparatus according to claim 6 wherein said meanscomprises a fuel separator having an inlet connected to said collectorpipe.

8. Combustion apparatus according to claim 5 having means defining anair entry to said combustion zone and means for swirling the air flowingthrough said entry about the axis thereof.

9. Combustion apparatus according to claim 8 comprising means defining aconstriction at the entry to the combustion zone.

10. Combustion apparatus according to claim 5 wherein said combustionzone is constituted by a vortex combustionchamber defined by two sidewalls, one of which is formed with a central axial outlet, and aperipheral wall formed with a generally tangentially facing inlet, and agenerally tangentially extending pre-combustion chamber leading to saidinlet, the vortex combustion chamber defining a spiral vortex path fromsaid inlet inwardly to said axial outlet.

11. Combustion apparatus according to claim 10 further comprising meansfor introducing further air into the vortex combustion chamber in theregion of said outlet.

12. Combustion apparatus for burning ash-forming liquid fuel comprisingmeans defining a combustion zone; a duct leading at one end to saidcombustion zone; a fuel injector mounted within said duct, said injectorbeing of a type which discharges said fuel as a spray of droplets ofvarious sizes; means for separating from said spray substantially allthose droplets of sizes less than a predetermined size, which separateddroplets would, under the conditions of combustion prevailing in saidzone, be substantially completely burned; means for removing saidseparated droplets from the duct; means for carrying the remainder ofthe droplets into the combustion Zone; an igniter for igniting saidremainder of the droplets in the combustion zone; said zone being shapedand dimensioned so that said ignited droplets are only partially burnttherein so that a proportion of unburnt carbon associated with fuel ashis formed, and having an outlet for combustion gases.

13. Combustion apparatus for burning ash-forming liquid fuel comprisingmeans defining a combustion zone; a duct leading at one end to saidcombustion zone; a fuel injector mounted within said duct, said injectorbeing of a type which discharges said fuel as a spray of droplets ofvarious sizes; means for directing a fluid stream in a directiondififerent from that of and into contact with said spray at such avelocity in relation to the mass, dimensions and velocity of thedroplets as to separate and carry away therefrom substantially all thosedroplets of sizes less than a predetermined size; means to receive saidfluid stream together with the droplets carried away thereby; means forcarrying the remainder of the droplets into the combustion zone; anigniter for igniting said remainder of the droplets in the combustionzone; said zone being shaped and dimensioned so that said igniteddroplets are only partially burnt therein so that a proportion ofunburnt carbon associated with fuel ash is formed, and having an outletfor combustion gases.

References Cited in the file of this patent UNITED STATES PATENTS2,374,606 McCollum Apr. 24, 1945 2,680,951 Winter et a1 June 15, 19542,828,608 Cowlin et al Apr. 1, 1958 FOREIGN PATENTS 1,095,032 FranceMar. 26, 1955 435,679 Great Britain Sept. 25, 1935 719,380 Great BritainDec. 1, 1954

